1. Field of the Invention
The present invention relates primarily to medical diagnostic equipment generating RF fields and using directional ultrasonic detectors, it can also be applied to characterization of materials and components in non-medical fields.
While the present invention is described herein with reference to illustrative embodiments for particular applications, it would be understood that the invention is not limited thereto. Those having ordinary skill in the art and access to the teaching provided herein will recognize additional modifications, applications, and embodiments within the scope thereof and additional fields in which the present invention would be of significant utility.
2. Description of Related Art
Radio frequency (RF) electromagnetic (EM) fields are widely used for medical diagnostics in magnetic resonance imaging (MRI). In MRI, RF is used to excite coherent precession of nuclear magnetic moments in a strong static background magnetic field. These processing moments then provide imaging information through the RP fields that they generate. Imaging occurs through detection of the subsequent re-emission RF fields that depend on the local densities of moments, their relaxation times, and precession frequencies. Good spatial resolution is achievable (5 micrometers).
Ultrasound has been used to characterize the structure and quality of tissue in a volume imaged by MRI. (U.S. Pat. No. 4,543,959 issued 1985 to Seponen.) MRI is used to sense organ motion produced by transverse acoustic waves launched into the patient to characterize tissue properties. (R. Muthupillai, D. J. Lomas, P. J. Rossman, J. F. Greeleaf, A. Manduca, R. L. Ehman xe2x80x9cMagnetic Resonance Elastography: Direct Visualization of Propagating Acoustic Strain Wavesxe2x80x9d Science 269 Sep. 29, 1995.)
Another approach to diagnostic imaging is supplied by Han Wen, Jatin Shah, and Robert S. Balaban xe2x80x9cAn Imaging Method Using the Lorentz Force of a Strong Magnetic Fieldxe2x80x94Hall Effect Imagingxe2x80x9d Pros, Soc. Magn. Reson. Med.xe2x80x9d Vancouver, B. C., p.279, May 1997. This latter approach requires application of electrical voltages to electrodes on the patient in a very strong steady (DC) magnetic field with detection of resulting ultrasound or the launching of intense ultrasound in the DC magnetic field with detection of the resulting voltage on electrodes.
The apparatus in all these cases is expensive and bulky because of the large DC magnetic fields used. Because of the cryogenics needed to support the DC field currents, the machines are awkward for use in remote locations.
Microwave EM imaging without magnetic resonance has been considered for medical diagnostics (xe2x80x9cMedical Application of Microwave Imagingxe2x80x9d, L. E. Larson and J. H. Jacobi Eds. IEEE Press, N.Y., 1985.) Excellent image contrast is produced because of the large variation of microwave refractive index among soft body tissues. However, these large variation deflect the paths of the microwaves traversing the body and thus distort the image produced. This effect also causes variable concentration of microwave power dosage in organs.
The injection of electrical currents and observation of resulting voltages at electrodes fixed to a patient have been studied as an imaging method (xe2x80x9cElectrical Impedance Tomographyxe2x80x9d J. G. Webster Ed., Adam Hilger, Bristol and New York, 1990; xe2x80x9cEvaluation of impedance technique for detecting breast carcinoma using a 2-D numerical model of the torsoxe2x80x9d Radai M. M., Abboud S., Rosenfeld M., Ann NY Acad Sci Apr. 20; 1999 873:360-9]. These methods are particularly significant in view of findings that breast cancer tumors have electrical conductivity four times that of surrounding normal tissue (D. C. Barber and B. H. Brown, xe2x80x9cClinical and Physiological Applications of Electrical Impedance Tomographyxe2x80x9d University College London Press, London, 1993; B. Blad and B. Baldetorp xe2x80x9cImpedance spectra of tumor tissue in comparison with normal tissue; a possible clinical application for electrical impedance tomography,xe2x80x9d Physiological Measurement 17, A105 (1996); Holder D. S., et al. xe2x80x9cAssessment and calibration of a low-frequency system for electrical impedance tomography (EIT), optimized for use in imaging brain function in ambulant human subjects.xe2x80x9d Ann N Y Acad Sci. Apr. 20, 1999; 873:512-9.) Imaging of tissue conductivity from such measurements involves inverting an elliptic differential equation which smooths out source details at distant observation points. This process is unstable in the sense that a small change in the observations produces large changes in the computed image. (Margaret Cheney xe2x80x9cInverse Boundary Value Problemsxe2x80x9d Am. Scientist 85 pp 448-55 (1997)).
To avoid the application of electrodes to the body, alternating magnetic fields have been used to produce images from currents in a phantom and a human thorax during respiration (xe2x80x9cMagnetic impedance tomographyxe2x80x9d Ann N Y Acad Sci Apr. 20, 1999; 873:353-9, Tozer J. C., Ireland R. H., Barber D. C., Barker A. T.) A problem with magnetic detection at sub-microwave frequencies is its inherently low resolution, MRI avoids this problem because of the spatially-dependent resonant frequencies which define position.
The present invention consists of an apparatus and associated non-invasive method for imaging internal regions of interest without the confinement and expense of a strong DC magnetic field, without the geometric distortion inherent in microwave or magnetic imaging and without the use of voltage electrodes on the patient. The apparatus is comprised of a pulsed RF source driving current through one or more induction coils adjacent to a bed supporting the patient and a scanning directional ultrasonic microphone or hydrophone. The detected output of the hydrophone and values of its look angles are fed to a video memory used for image construction off line or on line. The method consists of applying a pulse of RF current to the coils, detecting and recording the resulting ultrasonic signals which appear at twice the applied RF frequency; images of volumes within the patient are constructed by use of an algorithm applied to the recorded data. An alternative version uses a small DC magnetic field in addition to the RF EM field. Signals are then detected at the applied RF frequency as well.